Wheelchairs for use by handicapped and disabled persons have been well known for many years. One traditional wheelchair has a seat with a back, and two large wheels positioned on either side of the seat, which a user of the wheelchair can rotate manually in order to propel the wheelchair. The axes of the large wheels are typically behind the center of gravity of the wheelchair and occupant. Two relatively small caster wheels are provided at the front to support and balance the wheelchair. Such wheelchairs occasionally have small anti-tip wheels on rigid mountings to the rear, to catch the wheelchair if it should start to tip backwards. Such a wheelchair is shown in U.S. Pat. No. 3,848,883 to Breacain.
Motorized wheelchairs of similar general configuration are known. An example of such a wheelchair is shown in U.S. Pat. No. 5,540,297 to Meier. This wheelchair has the large powered wheels mounted on sprung trailing arms, and has the rear anti-tip wheels mounted on the ends of the trailing arms.
Power wheelchairs are also known in which the drive wheels are positioned forward of the center of gravity, and the wheelchair normally rests on the drive wheels and on rear caster wheels. These include wheelchairs, known as “mid-wheel drive power wheelchairs,” in which the drive wheels are aligned close to the position of the center of gravity and in which anti-tip wheels are provided at the front, to support the wheelchair if it should tip forwards and to assist it in climbing curbs and ramps. Commonly-assigned U.S. Pat. No. 6,129,165 shows a mid-wheel drive power wheelchair in which the front anti-tip wheels are inter-linked to the drive wheel suspension.